Recently, I reported a security issue in the new Firefox Screenshots feature (fixed in Firefox 56). This issue is remarkable for a number of reasons. First of all, the vulnerable code was running within the Web Extensions sandbox, meaning that it didn’t have full privileges like regular Firefox code. This code was also well-designed, with security aspects taken into consideration. In fact, what I found were multiple minor flaws, each of them pretty harmless. And yet, in combination these flaws were sufficient for Mozilla to assign security impact “high” to my bug report (only barely, but still). Finally, I think that these flaws only existed due to shortcomings of the Web Extensions platform, something that should be a concern given that most extensions based on it are not well-designed.